LEG 110

The Northern Barbados Ridge

During Leg 110, ODP achieved the first ever penetration of the detachment surface separating two converging plates in a subduction zone. This decollement zone lies between the underthrusting ocean crust of the American Plate and the overthrusting Barbados Ridge at the eastern edge of the Caribbean Plate. At the deformation front, or contact of the Barbados Ridge and the Atlantic abyssal plain, upper Neogene sediments riding passively on the Atlantic crust are offscraped to form part of the accretionary wedge.

Six sites were drilled during Leg 110, Site 671 to Site 676. Fluid migration and structural fabrics developed along the level of the future decollement were discovered 6 km east of the deformation front (Site 672). Low-angle reverse faults and mud-filled en-echelon arrays of veins in a lower Miocene interval that is equivalent to the decollement at the base of the accretionary prism are interpreted as structural characteristics of an incipient shear zone.

Three sites were also drilled at 5, 2, and 0.5 km arcward of the deformation front. (Sites 671, 675, and 676, respectively). At Site 671, ODP achieved the first ever penetration of the detachment surface, a 40-m-thick decollement zone in lower Miocene to upper Oligocene strata. The sedimentary deposits cored at these three sites are similar in thickness and lithology to those recovered at the oceanic reference site. Pore-water anomalies in chloride and methane content are concentrated mostly along the decollement zone. Dewatering of the inner part of the prism causes fluid flow along fault zones where the original low permeability of clayey sediments has been increased by fracturing.

At two sites located 12 and 17 km arcward of the deformation front (Sites 673 and 674, respectively), the intensely deformed middle Eocene-upper Pliocene accreted sediments display upright and overturned bedding related to initial thrusting and folding at the deformation front and subsequent folding and out-of-sequence thrusting along low angle faults that developed later. The occurrence of Eocene and Oligocene rocks in the accreted complex demonstrates that the decollement was at some stage in a deeper stratigraphic position. Evidence of fluid movement along discrete fault zones is provided by anomalous pore-water ion and methane concentrations and borehole temperature measurements that show unusually high values for corresponding depths.